Web, PostgreSQL, C

Hi All,

Sorry for stupid questions but all this stuff is new for me.
I am working on a CGI program to access Postgres DB from the Web.
I would like to use C for CGI.
I took standard post-query.c program that transfer data from Web to the
program.
Then I add some Postgres staff and compile the program with Postgres
libraries. And the program doesn't work. It dives this error:

Internal Server Error
The server encountered an internal error or misconfiguration and was
unable to complete your request.

Even more... If I compile (by gcc) post-query.c program adding to -I, -L,
-l options Postgres include and lib directories, the program gives the
same error.
What is wrong? Do I need to install something or change configuration
somewhere? 
I have PostgreSQL 6.5

Any help will be greatly appreciated.
Thank you,
-Margarita